From Dr David Harper, Senior Lecturer in Evolutionary Biology, University of Sussex
“Two problems for trees, one for people:
1. On all trees, bark exists in part to protect tree from infection: the pin holes break this barrier.
2. On small trees the pin holes may penetrate vital fluid translocation vessels.
3. Many pins end up not being removed by the polluters and may be incorporated into the bark. They are then extraordinarily dangerous to chain saw users; sadly many street trees end up being cut.
There is IMHO no excuse for it: to advertise your fete or neglected pet with a poster there are so many alternatives ranging from fixing them to posts to string (or duct tape).”